<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 200px;
            height: 200px;
            background-color: pink;
        }
        
        .change {
            background-color: blue;
        }
    </style>
</head>

<body>
    <div></div>
    <script>
        var div = document.querySelector("div");
        div.onclick = function() {
            // this.style.backgroundColor = "blue";
            // this.style.width = "400px";
            this.className = "change"
        }
    </script>
</body>

</html>